support empty date and time submissions for #13
support empty date and time submissions for #13

- caffeine.forms:
  - import timezone
  - rename timezone variable to cleantimezone
  - make date and time in SubmitCaffeineForm optional
  - react on empty date or time by using server time converted to user's
    timezone

coffeestats/coffeestats-django,coffeestats/coffeestats-django,coffeestats/coffeestats-django,coffeestats/coffeestats-django